1. Field of the Invention
The invention relates to microwave devices employing resonant elements made from dielectric material with a specific composition of the BaO - TiO.sub.2 system. Exemplary devices are pass-band filters and stop-band filters.
2. Description of the Prior Art
A variety of electrical devices use dielectric materials of various properties for various purposes. For example, materials with moderately high dielectric constants are used in such devices as dielectric resonator filters, microwave stripline circuits, various types of oscillators, as well as phase shifters, to name but a few. Dielectric constant is an important variable in the design of such devices, but equally important are low loss and temperature stability. For one class of devices, low loss is necessary to prevent dissipation of the electrical signal and for the design of circuits with high Q and narrow bandwidth. Temperature stability is required to prevent frequency changes in these devices. Good temperature stability permits much closer control of frequency characteristics when external temperature stabilization is used and may eliminate need for such stabilization in some applications. In addition, external temperature stabilization may not correct for temperature changes due to microwave heating of the dielectric material.
Up to the present time it has been relatively easy to obtain materials with high dielectric constant and low loss, but such materials have been accompanied by relatively large temperature variation of the dielectric constant. This temperature variation is measured by a temperature coefficient which represents the change in dielectric constant in parts per million per degree centigrade (ppm/degrees C). It should be recognized that device characteristics, such as frequency, etc., are particularly sensitive to slight variations in dielectric constant. This problem is usually more serious at high frequencies such as the microwave region (0.5 GHz to 200 GHz) as is the problem of low loss. For this reason, dielectric materials suitable for use in the low frequency region (below approximately 100 mc) have not been completely satisfactory in the microwave region.
The temperature coefficient of interest here is the one determined by changes of resonance frequency of a dielectric resonator. This effective temperature coefficient includes thermal expansion effects as well as dielectric effects. The effective temperature coefficient is defined by the equation: EQU .tau..sub.eff = -2/f df/dT
in which f is the resonant frequency.
Presently known dielectric materials do not have widespread use in dielectric resonator devices in the microwave region at this time. For example, one of the better materials with 20 mole percent BaO and 80 mole percent TiO.sub.2 (represented by the formula BaTi.sub.4 O.sub.9) has a dielectric constant of 38.0, a Q equal to 2500, but a temperature coefficient (of dielectric constant) of -49 ppm/degrees C at 2-11 GHz D. J. Masse et al, Proc. IEEE, November 1971, page 1628). Although dielectric constant and Q are satisfactory for microwave applications, the temperature coefficient is sufficiently high so that very close temperature control would be required to stabilize device characteristics. Similar results were obtained with a second material with composition 14.0 mole percent BaO and 86.0 mole percent TiO.sub.2 exhibiting a dielectric constant of 50.3, a Q of 1600 and a temperature coefficient (of dielectric constant) of -368 ppm/degrees C at X band frequencies (8-12 GHz, D. W. Readey et al, "Microwave High Dielectric Constant Materials," final report on Contract No. DAABO7-69-C-0455, reports control symbol OSD01366).